The Blogger’s will never know. That’s too bad. In most cases it was as simple as "you don’t post often enough. 12 posts a year won’t keep me interested".
Google Reader (and other readers) could fix this – when I select Unsubscribe they could let me enter a short reason why. Or not. Anonymous, or not. That reply could be emailed to Blogger’s that "opted in" for notification.
Heck – the same thing could be done when someone subscribed to a blog. "I liked your post about wet rabbits" or whatever.
So I am back down to under 400 blogs now. Ones I read every day, or many times a day I follow in Google Reader (since I can access them from any computer). Other less often read blogs I read in FeedGhost.